Recently, in line with miniaturization, lightweight, thin profile, and portable trends in electronic devices according to the development of information and telecommunications industry, the need for high energy density batteries used as power sources of such electronic devices has increased.
Lithium secondary batteries, as chargeable batteries that may best meet the need, have been used in portable electronic devices and communication devices, such as small video cameras, mobile phones, and notebooks.
In general, a lithium secondary battery is composed of a positive electrode, a negative electrode, and an electrolyte, in which charge and discharge may be possible because lithium ions, which are discharged from a positive electrode active material by first charging, may act to transfer energy while moving between both electrodes, for example, the lithium ions are intercalated into a negative electrode active material, i.e., carbon particles, and deintercalated during discharging.
Since there is a continuous need for high-capacity batteries due to the development of portable electronic devices, research into high-capacity non-carbon-based negative electrode active materials, which have significantly higher capacity per unit mass than that of carbon that is used as a typical negative electrode active material, has been actively conducted. Among these negative electrode active materials, it has been reported that a silicon-based negative electrode active material is inexpensive and is a high-capacity negative electrode active material having high capacity, for example, discharge capacity (about 4,200 mAh/g) about 10 times that of graphite as a commercial negative electrode active material.
However, since the silicon-based negative electrode active material is an insulator and the degradation of battery performance occurs due to a rapid volume expansion during a charge and discharge process accompanied by various side reactions, for example, crushing of negative electrode active material particles occurs, an unstable solid electrolyte interface (SEI) is formed, or capacity is decreased by electrical contact, this has been a great constraint in the commercialization of the silicon-based negative electrode active material.
Recently, in order to minimize the crushing of the silicon-based negative electrode active material due to charge and discharge, a technique of preparing a nano-sized silicon-based negative electrode active material has been proposed.
However, in order to prepare the nano-sized silicon-based negative electrode active material, a silicon-based material lump is prepared and the lump is then subjected to a nanoscale milling process, wherein, in this case, it may not be easy to control crystallinity of nano-sized silicon-based particles. Also, since the silicon-based negative electrode active material is oxidized during the milling process, there is a limitation in that initial efficiency of a secondary battery is eventually reduced.
Thus, in order to address the above limitation, there is a need to develop a method capable of preventing oxidation during the preparation of the nano-sized silicon-based negative electrode active material.